1. Field of the Invention
The present invention relates generally to a magnetoresistive sensor for use in a magnetic read head. In particular, the present invention relates to a tunneling magnetoresistive (TMR) read sensor having an enhanced magnetoresistive response.
2. Related Art
Magnetoresistive read sensors, such as giant magnetoresistive (GMR) read sensors, are used in magnetic data storage systems to detect magnetically encoded information stored on a magnetic data storage medium such as a magnetic disc. A time-dependent magnetic field from a magnetic medium directly modulates the resistivity of the GMR read sensor. A change in resistance of the GMR read sensor can be detected by passing a sense current through the GMR read sensor and measuring the voltage across the GMR read sensor. The resulting signal can be used to recover the encoded information from the magnetic medium.
A typical GMR read sensor configuration is the GMR spin valve, in which the GMR read sensor is a multi-layered structure formed of a nonmagnetic spacer layer positioned between a synthetic antiferromagnet (SAF), which is generally known as a pinned layer, and a ferromagnetic free layer. The magnetization of the SAF is fixed, typically normal to an air bearing surface of the GMR read sensor, while the magnetization of the free layer rotates freely in response to an external magnetic field. The SAF includes a reference layer and a pinned layer that are magnetically coupled by a coupling layer such that the magnetization direction of the reference layer is opposite to the magnetization of the pinned layer. The resistance of the GMR read sensor varies as a function of an angle formed between the magnetization direction of the free layer and the magnetization direction of the reference layer of the SAF. This multi-layered spin valve configuration allows for a more pronounced magnetoresistive effect, i.e. greater sensitivity and higher total change in resistance, than is possible with anisotropic magnetoresistive (AMR) read sensors, which generally consist of a single ferromagnetic layer.
A TMR read sensor is similar in structure to a GMR spin valve, but the physics of the device are different. For a TMR read sensor, rather than using a spacer layer, a barrier layer is positioned between the free layer and the SAF. Aluminum Oxide (AlO) is commonly used as a material for the barrier layer. Electrons must tunnel through the barrier layer. A sense current flowing perpendicularly to the plane of the layers of the TMR read sensor experiences a resistance that is proportional to the cosine of an angle formed between the magnetization direction of the free layer and the magnetization direction of the reference layer of the SAF.
A pinning layer is typically exchange coupled to the pinned layer of the SAF to fix the magnetization of the pinned layer in a predetermined direction. The pinning layer is typically formed of an antiferromagnetic material. In antiferromagnetic materials, the magnetic moments of adjacent atomic planes are aligned in alternating directions and, thus, there is no net magnetic moment in the material.
An underlayer is typically used to promote the texture of the pinning layer consequently grown on top of it. The underlayer is typically formed of a ferromagnetic material and is chosen such that its atomic structure, or arrangement, corresponds with a desired crystallographic direction.
A seed layer is typically used to enhance the grain growth of the underlayer consequently grown on top of it. In particular, the seed layer provides a desired grain structure and size. A seed layer may also be combined with the underlayer, in which combined layer performs the functions of both the underlayer and the seed layer.
One principal concern in the performance of TMR read sensors is the ΔR (the maximum absolute change in resistance of the TMR read sensor), which directly affects the magnetoresistive (MR) ratio. The TMR ratio (the maximum absolute change in resistance of the TMR read sensor divided by the resistance of the TMR read sensor multiplied by 100%) determines the magnetoresistive effect of the TMR read sensor. Ultimately, a higher TMR ratio yields a TMR read sensor with a greater magnetoresistive effect (greater sensitivity) that is capable of detecting information from a magnetic medium with a higher linear density of data.
A key determinant of the TMR ratio is the spin polarization of the sense current passing through the barrier layer. The sense current consists of majority spin electrons (spin is in the same direction of the magnetization) and minority spin electrons (spin is in the opposite direction of the magnetization). A spin polarized current has an unequal population of majority and minority spin electrons. According to the Julliere model of the TMR read sensor, the magnetoresistive effect in a tunneling junction is significantly enhanced if the sense current is spin polarized. This is because the magnetoresistive effect is determined by ΔR/R=2PP′/(1−PP′), where ΔR/R is the TMR ratio, and P and P′ are the spin polarization ratios of the effective tunneling density of states on each side of the barrier layer. The TMR ratio reaches a maximum value for completely polarized tunneling density of states (P=P′=1).
Large signal output and reduced shield-to-shield spacing have made the TMR head attractive for high linear density application. However, one of the crucial factors strongly limiting the TMR sensor in recording head application is the large resistance at higher linear density, which results in large shot noise, thereby reducing the signal to noise ratio. Large resistance also increases switching time since the insulating barrier acts as a capacitor. Tremendous effort has been made to reduce the resistance-area (RA) product of the TMR sensor in the past years. The lowest reported RA product of a given TMR sensor is 5 Ωμm2. Therefore, there is a need in the art for a TMR effect element having improved characteristics; particularly having a high TMR ratio and low resistance-area product (RA) for achieving improved head output.
The present invention addresses these and other needs, and offers other advantages over current devices.